a) Field of the Invention
This invention relates to a magnetoencephalograph for detecting the very weak magnetic fields from human brain using a high sensitivity SQUID (Superconducting Quantum Interference Devices), and more particularly to a magnetoencephalograph which uses an oxide superconductor magnetic shielded vessel which repels magnetic fluxes by a phenomenon of superconductivity of Meissner effect to exclude outside noises and thereby make it possible to detect the very weak magnetic fields from different parts of the brain and preferably has a magnetic-field detection unit comprising a plurality of gradiometers or magnetometers arranged in a helmet-shaped configuration fitting over the head.
b) Description of the Prior Art
Recently, researches attempting to elucidate the brain mechanism or the causes of headache or to make possible the medical examination of the brain by measuring magnetic fields from the brain have become active. Though imaging of the inside of the brain by X-ray CT (Computed Tomography), MRI (Magnetic Resonance Imaging) and PECT (Positron Emission Computed Tomography) are used in clinical examinations, there are various problems in the resolution of images, the response time, the limit of the X-rays used, and the exposure of patients to X-rays. EEG (Electoencephalography) is another method of examining the inside of the brain and is widely used in the clinical examination, but this method also has problems in that the electrodes must be stuck on the surface of the head and that the signals are distorted by the cranial bones. On the other hand, MEG (Magnetoencephalography), a method of making a map of the magnetic field distribution on the surface of the brain, has great advantages that measurements can be conducted without contact leads and that this method is effective not only for locating diseased parts in the brain but also for elucidating the functions of the brain for its quick response. Since the magnetic fields from the brain, however, are as weak as 10.sup.-12 to 10.sup.-15 (Tesla) and in a very low frequency region of 0.1 to 10 Hz, a conventional magnetic shield using a ferromagnetic material is not effective enough to detect the signals. The inventors of this application have experimentally confirmed before that a magnetic shield using an oxide superconductor is effective for biomagnetic measurements (JAPANESE JOURNAL OF APPLIED PHYSICS, VOLUME 28, NO. 5, 1989, L813 and volume 29, No. 8, 1990, L1435).
In order to locate diseased parts in the brain by measurements of the magnetic fields from the brain, the magnetic field distribution of the brain must be known by detecting the magnetic fields from different parts of the brain by means of a multichanneled SQUID. To conduct such measurements in a superconducting magnetic shielded vessel, a very large superconducting magnetic shielded vessel capable of housing a cryostat to cool the SQUID fluxmeter must be fabricated, and the superconducting magnetic shielded vessel itself must be cooled in a cryostat. Fabrication of such a large vessel, if technically possible, is attended with much difficulty and a high cost. Further, if many detecting elements are arranged so as to cover the head and detect the magnetic fields in all directions, the magnetic-field detection unit becomes considerably large in diameter. Moreover, since the SQUID must be replaced frequently because of its unstable characteristic, the superconducting magnetic shielded vessel must have a large opening formed in the top end to insert the SQUID in or retract it from the vessel, if the SQUID and the magnetic-field detection unit are constructed in a single unit. Such a large SQUID opening allows external magnetic fields to creep deeper into the vessel and so a much longer magnetic shielded vessel is required, consequently causing a problem of increase of the size of the magnetic shielded vessel.